With the rapid growth of optical networks, variable optical attenuators (VOAs) play an important role in the implementation of long-distance communication and metro area networks. For example, variable optical attenuators diminish differences of optical power between wavelength channels in wave division multiplexing (WDM) optical networks, and maintain the signal-to-noise ratios of the channels. Additionally, variable optical attenuators are commonly used to provide gain equalization in optical amplifiers, or to monitor and distribute optical power of a cross-connected network dynamically in a dense wave division multiplexing (DWDM) system.
Mechanical variable optical attenuators adjust optical attenuations through controlling optical coupling efficiency by moving fibers, mirrors, polarizers, and so forth. Unfortunately, the substantial volume of mechanical variable optical attenuators is usually too large to miniaturize or integrate the variable optical attenuators into a module. Variable optical attenuators controlled by micro electromechanical system (MEMS) provide downsized arrays of variable optical attenuators, which can be applied to a miniaturized module. However, fabricating such variable optical attenuators is so complicated that producing the same on large scale is time-consuming. On the other hand, electro-optic (EO) attenuators or thermo-optic (TO) attenuators modify optical attenuations according to refractive indices of specific materials therein altered by imposing different electric fields or different temperatures, respectively. Hence, one of the utmost issues for manufacturing EO or TO attenuators is to supply adequate optical applied materials.
To meet the needs of growing metro area networks, higher levels of device performance and low-cost components are necessary for variable optical attenuators. Another trend for advancing variable optical attenuators is integrating the same into an optical integrated circuit (OIC), which is similar to forming an integrated circuit (IC) in the semiconductor industry. Consequently, relevant researchers and manufacturers are devoted to developing novel materials or processes, or to miniaturizing or integrating optical devices, so as to reduce cost and enhance the properties of variable optical attenuators.